Severe premenstrual syndrome (PMS) is a chronic and complex mood disorder that involves a constellation of mood, behavioral and physical symptoms linked to the menstrual cycle with sufficient severity to disrupt functioning for as much as several weeks each menstrual cycle. Five to ten percent of reproductive-age women meet strict diagnostic criteria, but up to 40 percent seek medical treatment for this disorder. The efficacy of serotonin reuptake inhibiting antidepressants (SKIs) is clearly demonstrated for severe forms of PMS in short-term treatment trials. However, there are no long-term studies of severe PMS, and it is not known how long medication should be continued following symptom relief, to what extent and how rapidly symptoms return after stopping medication, or whether there is any additional improvement with long-term treatment. The proposed study compares short-term treatment (4 months) and long-term treatment (12 months) of severe PMS with the SRI sertraline and double-blind switch to placebo to study the effects of discontinuing medication. The aims are as follows: 1) Compare between short- and long-term treatment the percent of subjects relapsed within 6 months after discontinuing medication; 2) Determine the time to relapse after discontinuing medication and compare the time between the short- and long-term treatment groups; 3) Compare symptom levels while on drug between the short- and long-term treatment groups; 4) Compare the trend over time in symptom scores during the placebo period between the short- and long-term treatment groups. It is important to systematically examine these questions in order to minimize the risks and costs of medication if it is not needed or to support using medication if it is needed to prevent relapse and its attendant distress and disruptions of daily functioning for the large numbers of women who suffer from the disorder.